1. Field of the Invention
The present invention relates to optical waveguides, and particularly to a method for making optical waveguides having refractive index gratings. More specifically, the invention is directed to a method for making an apodized Bragg grating impressed on a planar optical waveguide.
2. Technical Background
Communication systems now increasingly employ optical waveguides that, because of their high speed, low attenuation and wide bandwidth characteristics, can be used for carrying data, video and voice signals concurrently. Multilayer optical waveguiding structures are used to build integrated optical circuits that route and control optical signals in a optical fiber communication system. It is possible to produce polymeric optical waveguides and other optical devices which transport optical signals via optical circuitry or optical fiber networks. In optical communication systems, messages are transmitted at infrared optical frequencies by carrier waves that are generated using sources such as lasers and light-emitting diodes.
The operation of an optical waveguide is based on the fact that when a core medium which is transparent to light is surrounded or otherwise bounded by a cladding medium having a lower refractive index, light introduced along the core medium's axis is highly reflected at the boundary with the surrounding cladding medium, thus producing a light-guiding effect. Optical waveguides can be make from any material that transmits light. Examples of suitable waveguide materials are polymeric materials and silica doped with germanium. One method used to form an optical waveguide device involves the application of standard photolithographic processes. Photopolymers are of particular interest for optical applications because they can be patterned by photolithographic techniques which are well known in the art. Photopolymers also offer opportunities for simpler, more cost-effective manufacturing processes. Lithographic processes are used to define a pattern in a light-sensitive, photopolymer-containing layer deposited on a substrate. This layer may itself consist of several layers composed of the same or different polymeric materials having dissimilar refractive indices, to form a core, overcladding, undercladding and buffer layers or structures. In practice, most planar waveguide structures have a configuration wherein a buffer layer is applied to a silicon substrate, then an underclad is applied to the buffer, followed by application and patterning of a core layer, and followed finally by application of an overclad. In some instances, the buffer layer can serve as the underclad. If these multiple layers are not optimized, several problems may occur. These include high optical loss due to absorption of light by the substrate; high polarization dependent loss; if heating is performed for tuning or switching, the increase of temperature may alter the index of refraction in such a way as to push light at least partially out of the core where it can interact with the cladding and/or the substrate to produce a variety of unwanted interactions which can, for example, lead to loss; and if the waveguide incorporates a grating, secondary reflections or an unwanted broadening of the wavelength of the reflected signal may be observed.
There is interest in these optical communication systems because they offer several advantages over electronic communications systems using copper wires or coaxial cable. Optical communications systems have a greatly increased number of channels of communication, as well as the ability to transmit messages at much higher speeds than electronic systems. An important extension of these communication systems is the use of wavelength division multiplexing, by which a given wavelength band is segmented into separate wavelengths so that multiple traffic channels can be carried on a single installed line. This requires the use of multiplexers and demultiplexers which are capable of dividing the band into given wavelengths which are separate but closely spaced. Adding individual wavelengths to a wideband signal, and extracting a given wavelength from a multi-wavelength signal require wavelength selective devices, and this has led to the development of a number of add/drop filters. Since wavelength selectivity is inherent in a Bragg grating, those skilled in the art have devised a number of grating assisted devices for adding or extracting a given wavelength with respect to a multi-wavelength signal. Typical optical waveguides propagate waves by the use of the light confining and guiding properties of a central core and a surrounding cladding of a lower index of refraction. Since wave energy is principally propagated in the core, add/drop filters or couplers have been developed using Bragg gratings formed in the core region of a waveguide. However, in order to reduce coupling losses to cladding modes, it is preferable to form Bragg gratings in both the core and in the surrounding cladding regions. Wavelength selectivity is established by the embedded grating which provides forward or backward transmission of the selected wavelength, depending on chosen grating characteristics.
For modern communication systems, however, this approach has a number of functional and operative limitations, pertaining to such factors as spectral selectivity, signal to noise ratio, grating strength, temperature instability and polarization sensitivity. For example, modern applications require that any add/drop filter based upon this concept be very efficient at routing channels, have a strong grating which can be selectively and precisely placed at or adjusted to a specific wavelength and yet have a limited bandwidth, be temperature insensitive, compact, low cost, and not subject to spurious reflections or noise in the chosen wavelength band.
Photosensitive waveguide materials cause the refractive index of that material to be susceptible to increase upon exposure to actinic radiation. Hence, a preferred method of writing a grating refractive index profile involves exposing a waveguide to a laser beam through a phase mask. Optical waveguide refractive index Bragg gratings are periodic or aperiodic variations in the refractive index of a waveguide. Gratings may be formed by physically impressing a modulation on the waveguide or by causing a modulation of the refractive index along the waveguide using photolithographic or other methods known in the art. Gratings written into the core of a waveguide or into the core and surrounding cladding of a waveguide are important components for many applications in optical fiber communication and sensor systems. To automate the fabrication process, it is desirable to write this refractive index profile into a waveguide in a single process step, i.e., with a single pass of the laser beam over the waveguide.
An optical waveguide provided with a Bragg reflection grating of uniform refractive index modulation, uniform pitch and high (>90%) peak reflectance has a spectral width directly proportional to the amplitude of the refractive index modulation. The main peak in the reflection spectrum of a grating with uniform modulation of the index of refraction is accompanied by a series of sidelobes at adjacent wavelengths. The sidelobes are caused by partial reflection of adjacent wavelengths and are undesirable. The side-band level can be reduced by apodization of the grating modulation amplitude in such a way that the strongest refractive index modulation occurs at the center of the grating, with the modulation amplitude decaying smoothly away to a low value at each end of the grating. Apodization reduces the level of the out of band reflectance to achieve suppression of the ghost images or sidelobes.
Apodizing the grating, thereby lowering the intensity of the sidelobes, is desirable in devices where high rejection of nonresonant light is required. In most of these applications, one desires that the apodization process also keep the average index of refraction constant across the grating length, which is sometimes difficult to achieve in a single-step process by controlling only the laser beam. Variation of the index modulation by changing the ultraviolet exposure along the length of the grating causes the magnitude of the refractive index modulation to vary and may cause the average photo-induced refractive index to vary. The average index variation leads to undesirable effects on the resonant wavelength of the grating and widens the grating spectral response. Keeping the average index of refraction constant during the apodization process is especially difficult if the waveguide material is silica doped with germanium. To alleviate these symptoms, after apodizing the grating to generate the non-uniform refractive index modulation, it may be necessary to apply a second compensating exposure to insure that the average photoinduced refractive index is constant along the length of the waveguide. One prior approach to created the desired apodization profile and uniform average refractive index has been by dithering the waveguide to decrease refractive index fringe visibility at specified locations along the waveguide length, but these techniques require complex mechanical fixtures for the phase mask and waveguide that can be vibrated yet precisely positioned.
The present invention provides a simple solution to the apodization problem by exposing the photosensitive waveguide through a phase mask to a laser beam scanned at an angle of more than 0° and less than 90° to the longitudinal axis of the waveguide. Either the laser beam is moved at a constant velocity with respect to the substrate and phase mask, or the substrate and phase mask are moved at a constant velocity with respect to the laser beam. The beam has a smoothly varying intensity profile (for example, a Gaussian profile), and the exposure is conducted under conditions to induce a change in the index of refraction to the waveguide and impart an apodized Bragg grating in the waveguide corresponding to the phase mask pattern.